Methane Adsorption in Anthracite Coal under Different Pressures and Temperatures — A Study Combining Isothermal Adsorption and Molecular Simulation

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Introduction
Coalbed methane (CBM) is clean energy and has been widely extracted and utilized with huge potential in China [1][2][3][4][5].The accumulation mechanism of CBM is quite different from other types of natural gas as CBM is mainly stored as absorbed state [6][7][8].Thus, methane adsorption mechanism in coal is considered to be one of the most important factors to realize CBM production [9][10][11][12].In previous studies, it was found that the adsorption behavior of methane in coal is caused by intermolecular interaction forces between methane molecules and coal matrix [11,13].In addition, the coal maturity, composition, pore structure, pressure and temperature all have effect on the methane adsorption capacity [8,[14][15][16][17][18].However, the pore system in coal is complex, and this makes the methane adsorption mechanism in coal is still unclear [19][20][21].
Methane adsorption isothermal experiment (gravimetric method or volumetric method) is commonly used to test the methane adsorption capacity of coal reservoir [15,22].The adsorption behavior has been widely studied under different pressure and temperature conditions [23,24].It was found that with increasing methane pressure, the adsorption capacity increases, which can be simulated by the Langmuir equation [23,25,26].Besides, with increasing temperature, the methane adsorption capacity decreases.When the vitrinite reflectance of the coal sample increases from 0.5% to 3.7%, the methane adsorption capacity decreases firstly and then increases [17,24,27].In general, the anthracite coals have strong adsorption capacities [24,28].The insitu CBM contents in many anthracite coalfields were found much higher than other coal seam [29].
Molecular simulation has been applied in studying the methane adsorption mechanism as molecule simulation could provide molecule-scale information about the methane adsorption behavior in coal [30][31][32].The density distribution of adsorbed methane, the location of the adsorbed methane, and the methane adsorption behavior of different size pores in coal can be directly provided by the molecule simulation [13,33,34].Therefore, the molecule simulation has received extensive attention in coalbed methane studies.The graphite-slit pore models are used to represent the adsorption behavior of methane in slit pores [13].However, the coal molecule structure is significantly different from graphite [32,35].
In the studies of CBM, it is important to evaluate the in situ methane content in coal.As the depth of the coal is different in different coalfields, the pressure and temperature of underground coal seams are different.The increase of burial depth of the coal seam can lead to temperature and pressure increment.However, it is unclear how temperature and pressure affect methane adsorption behavior.In this study, the influence of pressure and temperature on methane adsorption behavior is investigated by both physical experiments and molecular simulation methods.

Methodology
2.1.Samples.The coal samples used in this study are collected from No. 2 coal seams in Yuwang coal mine in East Yunnan (Diandong coalfield), China.Yuwang coal mine is an underground coal mine with high gas content.The average thickness of No. 2 coal seams is 1.13 m.The vitrinite reflectance of these coal samples is about 2.4%.The ash content of the coal sample is 23.66% and the volatile content is 9.99% under air-dry basis.

Experiments
2.2.1. 13C Nuclear Magnetic Resonance ( 13 C NMR).The 13 C NMR experiment was conducted by JNM-ECZ600R in Nuclear Magnetic Resonance Center of Tsinghua University.The powder coal samples were used and resonance frequency was set to 150 MHz.During the experiment, the mass frequency was set to 12 kHz with the total scan time of one hour (1200 times) [36].

Fourier Transform Infrared Spectroscopy (FT-IR).
When performing the FT-IR experiment, PerkinElmer spectrum (Frontier FT-IR) was used.About 2 g powder coal samples were used in the experiment.Before the experiment, atmospheric correction was performed.The scanning range was set from 450 cm -1 to 4000 cm -1 .The scanning speed is 0.2 s -1 .The analysis of the FT-IR is based on the previous study [37].

Methane Isothermal Adsorption
Experiment.The Methane isothermal adsorption experiment was performed in Unconventional Natural Gas Laboratory, China University of Mining and Technology, Beijing.Before the experiment, the Yuwang coal samples were pulverized to about 60-80 mesh and were dried at 80 °C for 24 hours.Methane adsorption experiments were carried out by the 3H-2000PH methane adsorption instrument by volumetric method.For the instrument, the pressure can be set to as high as 690 bar (69 MPa).

Coal Molecule Reconstruction.
The reconstruction process of coal molecular model contains several steps.Firstly, the molecular information of the model was obtained by statistical analysis of the experimental data such as industrial analysis, elemental analysis, 13 C NMR, and FT-IR.Then, the initial model satisfying the structural information is drawn by ACD/ChemSketch.After that, the 13 C NMR of initial model was calculated and compared to the experimental data.The initial model wall was corrected based on the comparing results to obtain the objective two-dimensional model structure.The two-dimensional model will be imported for model optimization to obtain the three-dimensional model with the lowest energy configuration.Finally, the periodic boundary is added to truly restore the microstructure of coal samples.The details of the molecule reconstruction process can be also seen in previous studies [38,39].

Simulation of Methane Adsorption in Coal.
The methane adsorption in coal is physical adsorption, and the interaction forces are mainly Van de Waals forces.In the simulation, the Van de Waals forces are calculated by Lennard-Jones potential energy function.The simulation was carried out at different temperature and different pressure.The maximum pressure is 7 MPa.The temperature and pressure conditions are set according to the experimental conditions. 13C NMR.According to the previous studies, the 13 C NMR spectrum of the Yuwang coal sample can be divided into four parts: aliphatic carbon peaks (chemical shift = 0 − 60 ppm), ether oxygen carbon peaks (chemical shift = 60 − 90 ppm), aromatic carbon peaks (chemical shift = 100 − 165 ppm), and carbonyl and carboxyl carbon peaks (chemical shift = 165 − 250 ppm) [40,41].Peak fitting of the 13 C NMR spectrum was performed by using the software origin.The fitting results were shown in Table 1 and Figure 1.

Results of
According to the peak fitting results, the carbon skeleton structure of YW coal sample is mainly composed by aromatic carbon structure (72%), followed by aliphatic carbon structure (14.2%).Carbons connected to the oxygen atoms contribute 13.7% of the total carbons in coal molecule.In addition, the aromatic carbon structure is mainly composed by benzene rings and naphthalene rings.There are less anthracene rings and phenanthrene rings.The structures of aliphatic carbon on the side chain are mainly composed by methyl and methylene.The oxygen-containing functional groups are mainly ether bonds.According to elemental analysis and 13 C NMR data, the aromaticity rate is 74.5%, and the total carbon in coal molecular structure unit is 200.The ratio of benzene : naphthalene : anthracene : phenanthrene : pentacene in YW sample is 5 : 5 : 3 : 0 : 2 (Table 2).

Results of FT-IR.
The peak fitting results of FT-IR are shown in Figure 2. The assignment of each peaks are according to Tables 3-6.The region between 2800 and 3000 cm -1 of FT-IR spectrum is the aliphatic carbon region.The infrared spectrum of this region shows that the side chain alkyl is mainly methylene and there is a small amount of methyl (Table 6).

Coal Molecular
Construction.The 13 C NMR and FT-IR data are used to reconstruct the chemical molecular structure of anthracite, and the average chemical formula of the anthracite coal molecule is C 200 H 133 O 21 N 3 , as shown in Figure 3(a).In addition, it can be seen that the oxygen atoms are mainly in the form of carbonyl groups, which can be also found from the 13 C NMR spectrum.The nitrogen atoms are in the form of pyrrole rings.After annealing and geometrically optimizing the 2D molecule, it can be found that the aliphatic side chain is obviously elongated and the aromatic carbon structure is distorted, as shown in Figures 3(b) and 3(c).Simultaneously, 14 optimized anthracite molecules put into the unit cell to construct a 3D structure of anthracite coal as shown in Figure 3(d).The size of the 3D molecular structure is 3:345 nm × 3:345 nm × 3:345 nm.   3 Geofluids of the YW coal sample under different temperatures.It can be seen that the adsorption capacity of methane increases significantly with increasing the pressure under the same temperature.This phenomenon demonstrates that it is conductive to the adsorption of methane under higher-pressure conditions.With increasing temperature, the methane adsorption amount is significantly weakened.At the pressure of 7.0 MPa, the methane adsorption capacity of the YW coal sample is 28.5 cm 3 /g under the temperature of 20 °C.Comparably, the methane adsorption amount under the temperature of 100 °C is only 15.9 cm 3 /g, decreasing by 44%.At the pressure of 1 MPa, with the temperature increases from 20 °C to 100 °C, the methane adsorption amount decreases from 10.0 cm 3 /g to 7.1 cm 3 /g.

Simulation of Methane Adsorption in Micropores of Coal
Sample.The pores of the model are calculated based on the 3D YW coal molecule, as shown in Figure 5.It can be seen from there are many pores in the coal matrix.These pores are very small in size, and the pore size ranges from 0.34 to 1.0 nm.In addition, the shape of these micropores is irregular.We employed methane adsorption simulation by using the 3D YW coal molecule and these methane molecules are adsorbed in the micropore (Figure 5).Figures 6(a) and 6(b) illustrate the simulation adsorption capacity of methane under different pressures and temperatures.With the methane pressure increasing from 1 MPa to 7 MPa at 100 °C, the absolute adsorption amount of methane increases from 7.52 cm 3 /g to 14.50 cm 3 /g, and the excess adsorption amount of methane increases from 7.30 cm 3 /g to 12.91 cm 3 /g.While, when the pressure reaches critical value, the absolute adsorption amount is found saturated, then the excess adsorption amount will show downward trend.Compared with pressure, temperature has a negative effect on methane adsorption.With temperature increasing, the adsorption capacity of methane gradually decreases.When the temperature is 20 °C, the absolute methane adsorption amount and excess methane adsorption amount is 25.2 cm 3 /g and 23.0 cm 3 /g under 7 MPa pressure.When the temperature increases to 100 °C, the absolute methane adsorption amount and excess methane adsorption amount decreases from 14.50 cm 3 /g to 12.91 cm 3 /g at 7 MPa pressure, decreasing by 42.4% and 43.9%, respectively.The adsorbed methane molecule is mainly located near the pore walls, and the methane adsorption density near the pore surface is much larger than that in the center of the slit pore.In addition, the methane density in the center of slit pore increases linearly with increasing the pressure.The density of adsorbed layers near the pore wall firstly increases significantly, and then increases slowly when the pressure is larger than 4 MPa. Figure 8(b) shows the methane density distribution at different temperatures.It can be found that with increasing the temperature, the methane density near the pore walls and in the center of the pore both decreases.However, the decrease of methane density in the center of pore is not obvious with increasing the temperature.Comparably, the methane density on the pore surface decreases significantly.It indicates that the temperature significantly affects adsorbed methane molecules.In Figures 8(a) and 8(b), the density near the pore surface is more sensitive to the pressure and temperate compared to the methane in the center of the pores.In addition, it can be seen that as the pressure increases, the methane density on both sides of the pore wall increases.While, as the temperature increases, the density of methane on both sides of the pore decreases.The reason is that the pores provide enough space in anthracite coal molecules, and as the pressure increases, more methane will be adsorbed on the pore surface.However, as the temperature increases, the methane molecules adsorb thermal energy and convert it into kinetic energy, thereby breaking the adsorption force of the pore wall and escaping from the pores.These simulation results are consistent with the experimental results of methane adsorption isotherm.

The Effect of Temperature on Methane Adsorption
Amount of Coal. Figure 9 shows the experimental methane adsorption data at different temperature.It can be seen that the methane adsorption amount of coal sample at 20 °C is significantly larger than those at larger temperature.Based   6 Geofluids on Figure 9, it can be seen that when the pressure is larger, the temperature has larger effect on the methane adsorption amount.At 7 MPa, the methane adsorption amount is 28.5 cm 3 /g under 20 °C, and decreases to 15.9 cm 3 /g under 100 °C, decreasing by 44%.However, at 1 MPa, the methane adsorption amount is 10.0 cm 3 /g under 20 °C, and decreases to 7.1 cm 3 /g under 100 °C, decreasing by 28%.
Experimental data illustrate the overall methane adsorption amount in all pores of coal sample including micropores, mesopores, and macropores.As shown in Figures 6  and 7, methane adsorption in micropores is different from that in mesopores.Thus, it should be discussed separately.Figure 10 shows the relationship of temperature and methane adsorption amount in micropores under different  7 Geofluids pressure.It can be found that at 20 °C the adsorption amount in micropores at different pressure is similar, but at 100 °C, the methane adsorption amount is micropores at lower pressure is obviously smaller than that at high pressure.It means, in micropores, temperature have larger effects on methane adsorption at low pressure than that at high pressure.
Figure 11 shows the influence of temperature on methane adsorption in mesopores under different pressure.With increasing the temperature, the methane adsorption amount decreases, which is consistent to the experimental data (Figure 9).It is also obvious that temperature has larger effect on methane adsorption amount at higher pressure than that at lower pressure.At 1 MPa, the  8 Geofluids methane adsorption amount at 20 °C is 0.01 cm 3 /g, and decreases to 0.0027 cm 3 /g at 100 °C, decreasing by 48%.Comparably, at 7 MPa, the methane adsorption amount at 20 °C is 0.066 cm 3 /g, and decreases to 0.014 cm 3 /g at 100 °C, decreasing by 78%.By comprehensively analyzing that the methane behavior in micropore, mesopore, and macropores, when the pressure is higher, temperature has larger effects on methane adsorption in mesopores and macropores, and has weaker effects on methane adsorption in micropores.The experimental data show  9 Geofluids that in YW coal sample, the temperature has larger effects on methane adsorption methane adsorption at highpressure stage, which is consistent to the phenomenon of methane adsorption in mesopore and macropores, and is contrary to the phenomenon of methane adsorption in micropore.It indicates that the mesopores and macropores in YW coal sample finally determine the temperature effect on methane adsorption.10 Geofluids

Conclusions
(1) Based on the 13 C NMR data and FT-IR data, the carbon skeleton of YW coal sample is mainly composed by aromatic carbon structure (72%), followed by aliphatic carbon structure (14.2%).Carbons connected to the oxygen atoms contribute 13.7% of the total carbons in coal molecule, and the oxygen atoms mainly exit in the form of carbonyl Comparably, less than 100 °C and 2 MPa, the methane adsorption amount is only 15.9 cm 3 /g, and decreases by 44%.Besides, the experimental data show that in YW coal sample, the temperature has larger effects on methane adsorption methane adsorption at higher pressure stage (4) In mesopores and macropores, temperature has larger effects on methane adsorption at higher pres-sure than that at lower pressure.On the contrary, in micropores, temperature has weaker effects on methane adsorption at higher pressure than that at lower pressure

Figure 1 :
Figure 1: 13 C NMR data of the YW coal samples with the peak fitting results.

Figure 2 :
Figure 2: The peak fitting results of FT-IR of YW coal samples.

Figure 4 :
Figure 4: Experimental methane adsorption isothermal data of YW coal samples under different temperatures.

Figure 6 :
Figure 6: Simulation methane adsorption under different temperatures ((a) absolute adsorption amount under different temperatures; (b) excess adsorption amount under different temperatures).

Figure 7 :
Figure 7: Simulation methane adsorption in 3 nm pore under different temperatures (a) absolute adsorption amount under different temperatures; (b) excess adsorption amount under different temperatures).

Figure 8 :
Figure 8: Characteristics of methane density variation in 3 nm slit pore at different pressures (a) and different temperatures (b).

Figure 10 :
Figure 10: The effect of temperature on methane adsorption amount in micropores (simulation data).

Figure 9 :
Figure 9: The effect of temperature on methane adsorption amount of coal (experimental data).

( 2 )( 3 )
The average chemical formula of the coal molecule is C 200 H 133 O 21 N 3 and the nitrogen atoms are in the form of pyrrole rings.By combining 14 coal molecules, the 3D macromolecular model was built and the size of the 3D macromolecular model is 3:345 nm × 3:345 nm × 3:345 nm The experimental methane adsorption isothermal data of the YW coal sample under different temperatures show that with increasing the temperature, the methane adsorption amount decreased obviously.At 7 MPa, the methane adsorption amount of the YW coal sample is 28.5 cm 3 /g under 20 °C.

Table 1 :
13C NMR peak attribution and relative content of YW samples.

Table 2 :
Forms of aromatic carbon with different molecular configurations of coal samples.

Table 3 :
[37]onal content of aromatic hydrocarbons (the assignments of the FT-IR is according to[37]).

Table 4 :
[37]onal content of oxygen-containing functional groups (the assignments of the FT-IR is according to[37]).

Table 5 :
[37]onal content of aliphatic carbon (the assignments of the FT-IR is according to[37]).

Table 6 :
[37]onal content of hydroxyl group (the assignments of the FT-IR is according to[37]).